io 6 ECHINOIDEA. I. 



reliable, as the small tridentate pedieellariee in the former group have also a simply serrate edge. 

 Other characters giving the same natural grouping of the species, do not seem to be found. 



The former group may be subdivided according to the ambulacral plates, the species csculentus, 

 acutus, and mclo having only a primary spine on every other ambulacral plate, while the species 

 elegans, gracilis, Alexandria ajfiuis, atlanticus, and lucidus have a primary spine on every ambulacral 

 plate. Thus this group might be subdivided into two genera according to this character. This divi- 

 sion, however, I do not think good; Ech. esculent as differs so much from acutus and melo, that it 

 seems to be incongruous to class it with these two species contrary to the other species of the group; 

 in quite young specimens of Ech. acutus a primary spine is often found on all the ambulacral plates, 

 which also tells against using this feature as a generic character. Finally it is also seen in the other 

 group that neither there a natural division can be obtained by means of this character. Thus it seems 

 to be correct to regard this whole group as one genus keeping the name of Echinus. The feature of 

 the ambulacral plates may here be used practically by the determination of the species. 



The other group, the species margaritaceus, Neumayeri, horrid us, utagcUanicus, and aldocinctus, 

 shows a series of striking peculiarities, so that the question naturally arises, whether all these species 

 are to be referred to one gemis. The characters by which a subdivision might be made, are, whether 

 every ambulacral plate or only every other plate has a primary spine, whether the secondary spines 

 are fine, silky, or not, whether or not the actinal spines are curved in the point, whether the buccal 

 membrane is quite naked, or fenestrated plates are found inside of the buccal plates; finally the 

 question might also be of using the pedicellariae or the features of the ocular plates as a basis of the 

 distribution of the species. 



E. aldocinctus is the most isolated one, especially distinguished by having only one unpaired 

 lateral tooth on the globiferous pedicellariae. As this feature, as will be shown below, is of very great 

 systematic importance, it seems reasonable to separate this species as a separate genus, even if in some 

 features it agrees very exactly with Ech. magellanictts (the quite naked buccal membrane, primary 

 tubercle on every ambulacral plate). For this form the name of Pseudechinus is proposed. - - To 

 separate the other four species is scarcely correct; according as one or other of the mentioned char- 

 acters is used as a base of the division we get a different grouping. Here a so curious intermingling 

 of all characters is found, that we only seem to have two chances left: to establish each species as a 

 separate genus — by which nothing is gained — or to unite them all to one genus, which latter I 

 think to be the most correct thing. Then this genus gets the name of Sterechiuus Koehler. Con- 

 sidering the common opinion of the difficulty of these species I shall give the following 



Table of the Stereehinus-speeies'). 



i. The secondary spines fine, silky 2. 



— — coarse 3- 



2. Primary tubercle only on every other ambulacral plate; the globiferous pedicel- 

 lariae with i — i lateral tooth, the edges connected by cross-beams St. margaritaceus. 



') A table of the Ec/ituus-species will be given below, after the description of the northern species. 



